Biomass and integrated forage/biomass yields of switchgrass as affected by intercropped cool- and warm-season legumes

Switchgrass (Panicum virgatum L.) has potential as a biofuel feedstock for ethanol production on marginal soils not suitable for row crop production. Further, it is hypothesized that legumes may be interseeded into switchgrass to increase available soil nitrogen (N) and enhance switchgrass yields. Therefore the primary objective was to identify compatible legume species for intercropping with lowland switchgrass and determine if biomass yields and forage quality can be improved. Four cool- and two warm-season legume species were compared to application of 67 and 134 kg N ha−1 (59.8 and 119.6 lb N ac−1) during 2009 and 2010 over a range of soils at three research and education centers in Tennessee. Cool-season legumes were alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), crimson clover (Trifolium incarnatum L.), and hairy vetch (Vicia villosa L.), and warm-season legumes included Illinois bundle flower (Desmanthus illinoensis L.) and partridge pea (Chamaechrista fasciculata L.). Legumes were evaluated for establishment (plant densities) and their effects on switchgrass yield and forage quality under a one-cut biomass (single, postdormancy biofuel) and an integrated two-cut (biomass/forage [preanthesis]) system. In the one-cut system, switchgrass yields (16.6 Mg ha−1 [6.7 tn ac−1]) from the current recommended rate (67 kg N ha−1 [59.8 lb N ac−1]) exceeded (p < 0.05) legume treatment yields (average 13.5 Mg ha−1 [5.5 tn ac−1]). In the integrated harvest system, switchgrass yields from red (13.4 Mg ha−1 [5.4 tn ac−1]) and crimson clover (12.8 Mg ha−1 [5.2 tn ac−1]) intercrops were not different from 67 kg N ha−1 (14.5 Mg ha−1 [5.9 tn ac−1]). Crude protein levels were greater (p < 0.05) for 134 kg N ha−1 (119.6 lb N ac−1), compared to legume intercrops (except red clover). Partridge pea showed promise as a warm-season legume that can be grown compatibly with switchgrass for up to two years. Therefore, compatible legume-intercrop candidates, such as partridge pea and red clover, may enhance switchgrass yield and forage quality while displacing synthetic N in integrated biofuel/forage systems, but need to be further investigated in efforts to reduce nitrate (NO3) leaching and emissions from fertilizing.

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